I. Field of the Invention
The current invention relates to position determination. More specifically, the present invention relates to a method and apparatus for improving error estimates of a position determination measurement made when determining the position of a device.
II. Description of the Related Art
It has always been desirable for man to know his geographic location. Devices ranging from compasses, maps, sextants, surveying equipment, etc. have been used to determine a person""s location. Today, we enjoy the benefits of a system of satellites that orbit about the earth and that provide information to receivers on earth. Each such receiver can use the information provided by the satellites to determine its position. One such system is the well know Global Positioning System (GPS). GPS is a xe2x80x9cconstellationxe2x80x9d of 24 well-spaced satellites that orbit the Earth. The accuracy with which GPS can determine the position of a GPS receiver is anywhere from 100 to 10 meters for most receivers. Each satellite within the constellation of GPS satellites transmits signals encoded with information. The information allows receivers on earth to measure the time of arrival of the received signals relative to an arbitrary point in time. This relative time of arrival measurement is generally referred to is a xe2x80x9cpseudo-rangexe2x80x9d measurement.
GPS is owned and operated by the U.S. Department of Defense, but is available for general use around the world. Briefly, GPS includes 21 xe2x80x9cregularxe2x80x9d satellites and three spare satellites in orbit at 10,600 miles above the Earth. The satellites are spaced so that from any point on Earth, at least four satellites will be above the horizon. Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, each satellite continually broadcasts its changing position and time. Once a day, each satellite checks its own sense of time and position with ground stations and corrects the information as necessary. On the ground, each GPS receiver contains a computer that xe2x80x9ctriangulatesxe2x80x9d its own position by getting bearings from three satellites for a two dimensional solution. The result is provided in the form of a geographic position. This position is typically in the form of longitude and latitude. The accuracy of the position determination is typically within 100 meters. If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map. If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position. If the receiver is moving, that receiver may also be able to calculate the speed and direction of travel of the receiver and give an estimated time of arrival to specified destinations.
Unfortunately, signals from GPS satellites are received at very low power levels due to the relatively large distances between the transmitting satellites and the receivers. Therefore, minimal obstructions in the signal path that either block or disperse the signal make it impossible for receivers to receive the signals. For example, most GPS receivers have great difficulty receiving signals inside a building, under dense foliage, in urban settings in which tall buildings block much of the sky, etc. Accordingly, other techniques are used in place of, or to supplement, GPS. One such system is commonly referred to as a xe2x80x9chybrid position determinationxe2x80x9d system.
A hybrid position determination system includes a position determination terminal that includes both a GPS receiver and a communication system receiver. In one example of such a hybrid position determination system, the communication system receiver is a cellular telephone receiver. A position determination beacon within the communication system communicates with the hybrid position determination terminal.
Signals from GPS satellites are received when available by the hybrid position determination terminal via the GPS receiver. xe2x80x9cAiding informationxe2x80x9d is received from the position determination beacon by the hybrid position determination terminal via the communication system receiver. The aiding information includes information that allows GPS satellite signals to be rapidly located in frequency and time. In addition, the communication system signals can also be used to determine pseudo-ranges to base stations, one or more of which may be a position determination beacon. The pseudo-ranges to the base stations are used together with the pseudo-ranges to the satellites to calculate the position of the receiver.
In addition, base stations provide a time reference to the position determination receiver within the hybrid position determination terminal. In one particular hybrid system, the time reference provided to the receiver by the communication system is GPS time. However, the GPS time that is provided is offset by the amount of time required for the signal communicating the GPS time to propagate from the position determination beacon to the position determination receiver. This offset can be determined by measuring the propagation delay encountered by a signal that is transmitted on a xe2x80x9cround tripxe2x80x9d from the communication system receiver to the position determination beacon and back to the communication system receiver. The offset is then equal to one half of the total round trip delay (RTD). However, it should be noted that there is a delay that is added to the RTD by the internal delays associated with the reception and retransmission of the signal at the position determination beacon. Therefore, in order to get an accurate GPS time transfer from the position determination beacon to the position determination terminal, these internal delays must be determined and subtracted from the measured RTD. This is often referred to as xe2x80x9ccalibratingxe2x80x9d the position determination beacon. Calibrating the position determination beacon requires measuring the amount of delay internal to the position determination beacon. Calibrating the position determination beacons is a time consuming and difficult task. Accordingly, it would be advantageous to provide a method and apparatus that would allow the position of a hybrid position determination terminal to be determined without requiring calibration of position determination beacons.
Even after having calibrated each of the position determination beacons within a communication system, the accuracy of the pseudo-range measurements that are made between the position determination terminal and the position determination beacon are not necessarily accurate. This is due to a phenomenon known as xe2x80x9cmultipathingxe2x80x9d. Multipathing occurs when a signal takes an indirect path between the transmitter (i.e., the position determination beacon) and the receiver (i.e., the position determination terminal). An indirect path is defined as a path that is longer than the shortest distance between the transmitter and receiver. The word multipathing implies that more than one signal path will be traversed by the signal between the transmitter and receiver. However, for the purposes of this discussion, a signal would still be considered to be a multipath signal, even if the signal takes only one indirect path between the transmitter and the receiver.
Multipathing increases the amount of time required for the signal to traverse the distance between the position determination beacon and the position determination terminal. This increase is due to the longer distance traveled by the signal as a consequence of reflections off obstacles, such as buildings. The increase in the amount of time required for the signal to arrive at the receiver results in an error in the pseudo-range measurement. This pseudo-range measurement error is then translated into an error in the position that is calculated from the pseudo-range measurements.
Multipathing can be a problem in GPS signals. However, it is easier to mitigate the effects of multipathing in GPS signals, since it is likely that the signal will still arrive at the position determination terminal via the direct path. That is, the signal between the GPS satellite and the position determination terminal is likely to take more than one path. However, one of those paths is likely to be the direct path. Accordingly, the direct path is assumed to be the one that is first to arrive. In addition, the direct path typically will have greater signal strength. In contrast, communication signals transmitted from a position determination beacon are more likely to take only indirect paths.
Therefore, there is a need to determine the error that is introduced by multipathing. The following description discloses a method and apparatus for determining an estimate of the amount of en-or that is present in pseudo-range measurements made in a hybrid position determination system.
The presently disclosed method and apparatus allows correlations between a selected parameter and the error in a pseudo range measurement to be exploited. A database is established in which the amount of error estimated for particular pseudo range measurements to a beacon is maintained. Clusters are defined. Each cluster is associated with a range of values for the selected parameter. Pseudo range measurements are then associated with a particular cluster based upon the value of the selected parameter at the time (or proximate to the time) the pseudo range measurement was taken. As more estimates of the pseudo range measurements are made, the size of the clusters (i.e., the range of values of the selected parameter) can be reduced. Due to the correlation between the selected parameter and the errors in the pseudo range measurements, reducing the size of the clusters reduces the variance of the error estimates. The mean value of the error estimates is used to correct the error in future pseudo range measurements.
In one embodiment of the disclosed method and apparatus, the position of a terminal that measures the pseudo range to a beacon is the selected parameter. Alternatively, any other correlated parameter, such as the power level of the beacon signal, may be the selected parameter. The size of the cluster is initially relatively large, since the database will have relatively few error estimates in any particular geographic region. However, as the number of error estimates increases, the size of the clusters can be decreased, thus reducing the variance of the error estimates within the smaller clusters with respect to the larger clusters.
In accordance with one embodiment, error estimates are made by first calculating what the pseudo range to a particular beacon should be. This calculation is made by determining (using a highly accurate first position determination sub-system) the current position of the terminal used to make the pseudo range measurements to the beacon. Once the position of the terminal is known, the pseudo range measurements to a beacon can easily be calculated, assuming that the location of the beacon is known. The pseudo range to the beacon from the terminal is then measured using a less accurate second position determination sub-system. The difference between the pseudo range that is calculated based on the more accurate first position determination sub-system and the pseudo range measurement made by the less accurate second position determination sub-system is determined. This difference is assumed to be due to the error in the measurement made by the less accurate second position determination sub-system.
Accordingly, the database includes information that allows pseudo range measurements made by the less accurate second position determination sub-system to be corrected when the more accurate first position determination sub-system is not available. The database is self generating, in that the information required in the database is taken during the operation of the terminal based upon the availability of the more accurate first position determination sub-system. The more points the terminal can locate using the more accurate first position determination sub-system, the smaller the clusters in the database. As a consequence of smaller clusters, the variance in the error estimates that are maintained in the database for each cluster will be reduced.
It should be understood that the presently disclosed method and apparatus could be used with position determination systems other than hybrid position determination systems if there is some other means by which to determine the position of the terminal and that means is available at some times or locations, but not at others. In that case, the position of the terminal would be used as a reference to determine the amount of error in the pseudo range measurements in the same way as the position determination from the more accurate sub-system described above.
In accordance with one embodiment of the disclosed method and apparatus, an iterative approach is used when the selected parameter is the location of the terminal. Assuming that the more accurate first position determination sub-system is not available, and that a sufficient number of initial error estimates have been made, the iterative approach uses corrected measurements of the pseudo range based upon a relatively large cluster to determine the position of the terminal. Once the location of the terminal has been determined in this way, the corrections to the pseudo ranges can be recalculated based upon a much smaller cluster, assuming that a statistically valid number of error estimates has been made for the smaller cluster.